Typically, a ringer circuit is provided in the telecommunication terminal equipments, e.g. telephone, facsimile machine, of the telecommunication network. When the telecommunication ring signal comes in, the terminal equipment generates a ringer audio signal in response informing the user to receive the message coming in.
The Federal Communication Committee (FCC) sets up a plurality of standards on the telecommunication equipments having a ringer circuit, which are to be followed by the industry. Among them is the Ring Equipment Number (REN) value of a telecommunication ring equipment, which is a measure of the impedance of the ring equipment. The detailed guideline and method of calculating REN of a ringer generation equipment may be found in 47 CFR Chapter 1, Section 68.312 and therefore will not be reiterated herein. However, generally speaking, as REN calculated is larger, it implies that the impedance of the ringer generation equipment is smaller and vice versa. The higher REN value an individual ringer generation equipment has, the smaller the number of ringer generation equipments allowed to be connected onto the telecommunication network is. FCC also regulates that the REN of a single telecommunication ringer generation equipment has to be smaller than 5. The prior art ringer circuit, as shown in FIG. 1, mainly includes a rectifier 11, a Zener diode 12, a capacitor 13 of a filter function and a ringer generation circuit 14. Furthermore, the capacitor c1 is provided to filter the direct current (DC) signal and to allow the passage of the alternating current (AC) signal. The resistor R1 is a current limiting resistor.
When AC ringer input signal V comes in via input terminals a, b, a DC voltage is generated at the output terminal H of the capacitor 13 to drive the ringer generation circuit 14. Since the ringer generation circuit 14 is easily damaged by a higher voltage, a Zener diode 12 is usually supplied to protect it. The ringer generation circuit 14 may be designed in a well known manner.
However, during the testing of the ringer circuit shown in FIG.1 under a high voltage ring input signal, e.g. 150 V/68 Hz, the Zener diode 12 is conducting and accordingly presents a lower impedance value. As a result, the REN obtained will be larger. Telecommunication equipment labelled with a larger REN value would have, in general, a negative influence on the sales of this equipment.
Under the aforesaid testing environment, a current limiting resistor R1 with larger resistive value has to be used in order to lower the obtained REN value. But, as this measure is taken and under a normal operation condition, e.g. an input ring signal with voltage between 40 V through 150 V, the DC voltage at H terminal will decrease. As a result, the ringer audio signal generated from the ringer generation circuit 14 and heard by the end user will be too small. In other words, if we adopt the circuit such as that in FIG. 1, requirement of lower REN value while generating a larger ringer audio signal heard by the end user can not be met at the same time.